This invention is directed generally to improvements in the testing of line screen CRTs (cathode ray tubes). It is particularly directed to a method and apparatus for detecting the degree of misregistration between a line screen CRT's electron beam and the phosphor stripes which the beam is intended to excite.
For a color CRT to develop an image of high brightness and high purity, the CRT's electron beams must impinge on the proper phosphor stripes. The extent to which the electron beams stray from their intended targets is referred to as misregistration.
The extent of misregistration, if any, depends upon a number of factors, but it is particularly dependent upon the quality of the light house lens which is used in the deposition of phosphor targets on the CRT screen. Developing a lens to achieve good registration has been a very time consuming process which is subject to human error.
One step in the development of a good lens involves measuring the misregistraton of a CRT made with a prototype lens, and then modifying the lens to eliminate or reduce the measured misregistration. Typically, the measurement of misregistration includes first constructing a CRT with oversize phosphor targets, energizing the CRT, and then photographing multiple areas of the CRT screen under electron beam bombardment. After developing the photographs, measurements are made on the photographs to determine the distance between the center of the electron beam and the center of its phosphor target. Based on those measurements, the prototype lens may be refined to reduce the CRT's misregistration, and another set of measurements may be made to confirm that the lens achieves its objective.
Two problems are associated with the conventional registration test described above. Firstly, it is very time consuming, expensive, and subject to human error. Secondly, it requires the use of a special CRT with oversize phosphor targets. It is desirable, therefore, to provide a method of measuring misregistration which is faster and which uses production type tubes to obtain more accurate registration information.